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Proxima Centauri,Alpha Centauri Binary Stars and the Sun
Proxima Centauri is classified as a red dwarf star because it belongs to the main sequence on the Hertzsprung–Russell diagram and it is of spectral class M5.5. It is further classified as a "late M-dwarf star", meaning that at M5.5 it falls to the low-mass extreme of M-type stars.[11] This star's absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs, is 15.5.[4] Its total luminosity over all wavelengths is 0.17% that of the Sun,[7] although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun.[35]More than 85% of its radiated power is at infrared wavelengths.[36] http://en.wikipedia.org/wiki/File:Alpha_centauri_size.pngThis illustration shows the comparative sizes of (from left to right) the Sun, α Centauri A, α Centauri B, and Proxima Centaurihttp://en.wikipedia.org/wiki/File:Alpha,_Beta_and_Proxima_Centauri.jpgThe two bright stars are (left) Alpha Centauri and (right)Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Taken with Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30 seconds. In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri was 1.02 ±0.08milliarcsec. Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter.[26] The star's estimated mass is only 12.3% of a solar mass, or 129 Jupiter masses.[11] The mean densityof a main sequence star increases with decreasing mass,[37] and Proxima Centauri is no exception: it has a mean density of 56,800 kg/m3 (56.8 g/cm3), compared with the Sun's mean density of 1,409 kg/m3 (1.409 g/cm3).[nb 2] Because of its low mass, the interior of the star is completelyconvective, causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end.[16] Convection is associated with the generation and persistence of amagnetic field. The magnetic energy from this field is released at the surface through stellar flares that briefly increase the overall luminosity of the star. These flares can grow as large as the star and reach temperatures measured as high as 27 million K[31]—hot enough to radiate X-rays.[38] Indeed, the quiescent X-ray luminosity of this star, approximately (4–16) ×1026 erg/s ((4–16) ×1019 W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 1028 erg/s (1021 W.)[31] The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm.[39] About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona.[40] However, the overall activity level of this star is considered low compared to other M-class dwarfs,[15] which is consistent with the star's estimated age of 4.85 ×109 years,[11] since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases.[41] The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.[42] Proxima Centauri has a relatively weak stellar wind, resulting in no more than 20% of the Sun's mass loss rate from thesolar wind. Because the star is much smaller than the Sun, however, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.[43] A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming from red to blue. Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity and warming up any orbiting bodies for a period of several billion years. Once the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.[16] The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven withmagnetic fields.[11][12] It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System.[13] Chemically, about three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. The remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.[14] The Sun's stellar classification, based on spectral class, is G2V, and is informally designated as a yellow dwarf, because its visible radiation is most intense in the yellow-green portion of the spectrumand although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.[15][16] In the spectral class label, G2 indicates its surface temperature of approximately 5778 K (5505 °C), and V'' indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.[17][18] The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is the brightest object in the sky with an apparent magnitude of −26.74.[19][20] The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.[21][22] The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf namedProxima Centauri at approximately 4.2 light-years away), the Sun ranks fourth in mass.[23] The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light-years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years. Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, the Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.[24] The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves from perihelion in January to aphelion in July.[25] At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supportsalmost all life on Earth by photosynthesis,[26] and drives Earth'sclimate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained. '''Alpha Centauri' (α Centauri, α Cen; also known as Rigil Kentaurus,Rigil Kent /ˈraɪdʒəl/, or Toliman) is the brightest star in the southernconstellation of Centaurus. Although it appears to the unaided eye as a single object, Alpha Centauri is actually a binary star system (designated'Alpha Centauri AB' or α Cen AB) whose combined visual magnitude of -0.27 would qualify it as the third single brightest star in the night sky after -0.72 magnitude Canopus and −1.46 magnitude Sirius. Its individual component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of our Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 50.0% of its luminosity. During the stars' 79.91 year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun. They average 1.34 parsecs or 4.37light years away from the Sun.[10] A third star, known as Proxima Centauri, Proxima or Alpha Centauri C (α Cen C), is probably gravitationally associated with Alpha Centauri AB. Proxima is now placed at the slightly smaller distance of 1.29 parsecs or 4.24 light years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The true separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000astronomical units (AU), equivalent to 400 times the size of Neptune's orbit.